This invention relates to a method and an apparatus for propelling pipes to be laid in the ground in a propulsion pipe-laying method. It particularly relates to a method and an apparatus for laying pipes of comparatively small diameters made of polyvinyl chloride and the like and, thereby, of lower bearing force, than steel pipes, rather than those for laying such pipes as a gas pipe, a drainage pipe, and pipes for other uses.
There have been laid a number of pipes of many kinds such as a gas pipe, a water pipe, and a drainage pipe in the ground. Since steel pipes, concrete pipes, Hume pipes, and the like have high bearing force, they are used as pipes of a large diameter to be laid in main lines. On the other hand, such pipes as vinyl chloride pipes and the like are not suitable for use in main lines because of their low bearing force, but they are popularly used as pipes of a relatively small diameter in branch lines because of their small weight, ease in handling, an, cheap manufacture cost and, in addition, compared with a case of the main ine use, the pipes in branch lines are in greater amounts of demand and exchanged more frequently.
As a method to lay these kinds of pipes in the ground, the propulsion pipe-laying method is recently becoming popular. An example of this method is described in U.S. Pat. No. 4,024,721. In this method, a vertical pit is at first excavated from the ground surface and then, a pipe of standard length is put down in the pit to be inserted from the pit into the ground in a horizontal direction. The insertion is carried out by jointing the pipes one after another until the pipes reach a required length. This method does not need to excavate an open trench in a length equal to that of the series of pipes to be laid, so that the traffic on the ground surface is not seriously distributed.
In performing this method, the pipes are propelled by a jack, arranged in the vertical pit and pushing at the end of a trailing pipe in the pipe series. At the head of a leading pipe, excavation means (for example, an auger) is installed to excavate a tunnel for pipe-laying.
In this propulsion pipe-laying method, the pipes being jointed receive great repulsion force generating between the auger etc. and the ground in addtion to friction power between the pipes and the surrounding ground. Accordingly, this method has been so far only applied in the works in main lines where pipes of a large diameter are used.
However, if the propulsion pipe-laying method can be applied in the works in branch lines, it would be convent and beneficial to the public. Under these circumstances, the present inventor previously invented a method disclosed in the Japanese Patent Gazette (Kokoku) No. 59-45077.
In this new method, an auger etc. being installed on the head of a propulsion shaft of the length equal to that of a pipe to be laid, the propulsion shafts as well as the pipes being jointed one after another, and the shafts being pushed by a jack in the vertical pit, the excavation means is propelled in the ground. In this method, a leading head pipe in a series of pipes is connected with the excavation means. By doing this, the group of pipes are inserted in the ground being drawn by the excavation means.
In this new method, the repulsion force receiving from the ground is transmitted to the propulsion shafts, but not to the pipes. Thus, the shafts are made of a firm material such as steel and the like, the pipes can be made of a material of low bearing force like vinyl chlolide.
However, there has been found a serious problem in the new method. A pipe to be laid receives friction power resulting from the surrounding ground and then, as the pipes are jointed one after another, the friction power being received by the jointed pipes is in turn transmitted to the pipes in a front side in the series of pipes and, as a result, in the series of pipes led by the excavation means installed at the leading pipe, the leading pipe will receive the greatest tensile stress. Accordingly, there is a problem that the leading pipe is often transformed or destroyed. In some instances, such transformation or destruction takes place at a pipe near the leading pipe in the series of pipes. As the pipe series become longer, this kind of problem is more often encountered.
Therefore, when pipes of low bearing force are laid according to the propulsion pipe-laying method, a total length or number of a series of pipes to be laid at one operaton will be cut so as to avoid huge propulsion power which may work at the head of the leading pipe in the series. However, even though the total length of pipes at one operation is cut, the vertical pit will be needed for every phase of operation in a series of pipes to be laid. Besides, it inevitably causes a necessity of moving and repeated installation of machinery such as a jack and the like which is of great annoyance and increases cost. A plurality of pits dug with a short interval of space on the ground surface will be a serious obstruction for traffic.